Amount of time to keep unavailable URLs (with bad status, e.g. "404 Not found" or "503 Service unavailable") before automatically deleting them from the database. For example, if remote host is down, indexer will not delete pages from this site immediately, and search will use previous content of these pages. However, if site doesn't respond for a long time (e.g. a month), probably it's time to remove these pages from the database.
The default value is 0, which means never delete unavailable URLs from the database - for better indexing speed. You can periodically delete These URLs manually, using indexer with -s command line parameter (status limit), for example: "indexer -Cw -s404".
For "time" format see description of the command Period.